The present invention relates to an antenna device and an electronic equipment having the antenna device, in particular to an antenna device contained in an electronic equipment which can be commonly used at a plurality of frequencies capable of being utilized in a radio communication.
In addition to Local Area Network(LAN)s which are widely used in desk-top type computers, wireless LANs, for example, the Bluetooth system, that can be used also in portable type computers have been spread in a computer network in recent years. As a specification of an antenna device for a radio communication used in such a portable type computer, the following items are required.
Namely, a multi-frequency operation, for example, one frequency band of 2.4 GHz and another frequency band of 5.2 GHz. Herein, in order that the portable type computer may be responsive to both the frequency bands, it has been conventionally required that the portable type computer has two kinds of antenna devices. However, it is difficult to obtain spaces for mounting the two kinds of antenna device, respectively, since the portable type computer is designed to be as small as possible in size and weight. Accordingly, it is required that even a single antenna device can be responsive to both the frequency bands so that the spaces for mounting the antenna device may be as small as possible in a portable type computer.
Further, since the portable type computer is designed to be as small as possible in size and weight, as mentioned above, it is preferable that the antenna device can be contained in the portable type computer. Accordingly, it is, of course, required that the antenna device is small in size. In addition, it is further required that the antenna device is not easily influenced electrically from an adjacent housing body, or the like.
For example, a wire antenna, such as a dipole antenna, a monopole antenna, and the like resonates at a frequency of integer times (one, two, three . . . ) as large as a predetermined frequency, in a case that the wire antenna has an antenna-length of approximately ninety-five percentages as long as a wave-length of as half as the predetermined frequency. However, between the two frequencies usable in the wireless LAN (hereunder called first and second frequencies), second frequency is not integer times (one, two, three . . . ) as large as first frequency, as mentioned before. As a result, concerning a conventional dipole antenna, or the like, a single antenna device cannot be responsive to both the frequency bands.
Accordingly, an example of a conventional antenna device is disclosed in unexamined Japanese patent publication Hei2-57003, namely, 57003/1990. In order to be responsive to both the frequency bands mentioned above, the conventional antenna device disclosed therein has two dipole antennas resonating at first, second frequency, respectively. The two dipole antennas are located in parallel in the same feed line and supplied with electric power transversely. However, in the conventional antenna device having two dipole antennas, not only a structure of the antenna device inevitably becomes large in size but also a constitution of an impedance matching section becomes complicated. Further, the conventional antenna device having the two dipole antennas is disadvantageous in actual use, since loss is increased in the feeder thereof, and the like.
On the other hand, an input impedance of a conventional dipole antenna, and the like becomes low almost down to short-circuit impedance near a metal conductor, particularly when an interval between the conventional dipole antenna and the metal conductor is not longer than a wave-length of one-tenth of the predetermined frequency. In addition, each of the first and the second resonant frequencies comes to a frequency characteristic having a narrow band. As a result, when the dipole antenna, and the like is contained in a computer, it becomes difficult to obtain impedance matching between an antenna element and a feeding system thereof. Further, it also becomes difficult to generally use the dipole antenna, and the like by way of a coaxial cable, and the like.
Accordingly, as an antenna device capable of being commonly used at the first and the second frequencies, a proposal is made about an antenna device in which a parasitic element resonating at the second frequency is additionally located in a dipole antenna resonating at the first frequency. For example, not only in unexamined Japanese utility model publication Sho62-191207, namely, 191207/1987 but also in unexamined Japanese patent publication Sho63-171004, namely, 171004/1988, disclosure is, respectively, made about an antenna device that a parasitic element consisting of a feed-less element is additionally located near a dipole antenna resonating at the first frequency, so that a resonant characteristic of the second frequency can be obtained in the antenna device.
However, the resonant characteristic of the second frequency is obtained in the antenna device by additionally locating the parasitic element, limitation is caused to occur for a position and a size of the parasitic element in the antenna device. Further, the antenna device becomes large in size by a size of the parasitic element. In view of a radiation characteristic of the dipole antenna, it is necessary for the antenna device to be separated from the adjacent metal conductor, and the like by a distance of a quarter wave-length of the first frequency approximately, and integer times as large as the first frequency in addition thereto. As a result, a space of not smaller than the wave-length of one-fourth of the first frequency is required for mounting the antenna device in the computer.
Under the circumstances, as an antenna device capable of being contained in a computer by readily obtaining impedance matching between an antenna element and a feeding system thereof, a proposal is made about an antenna device, such as a loop antenna, a folded dipole antenna, and the like, each of which is a wire antenna that an input impedance is increased by folding an antenna element.
However, in the above-mentioned antenna device that is a wire antenna, such as a loop antenna, a folded dipole antenna, and the like, a resonant frequency of the antenna device depends on an antenna length thereof. It is therefore difficult to adjust the second frequency after the first frequency has been adjusted.